Power supplies used in portable systems, such as Global System for Mobile Communications (GSM) mobile terminals, must be able to take advantage of the time varying nature of such systems. Typically, portable systems have an active state where the power supply is required to provide the full operating current (high power mode) and a standby state that requires minimal operating current (low power mode). For example, when the digital baseband engine in a GSM mobile terminal is actively processing data the power supply is required to provide the full operating current, but when the digital engine is not processing data the power supply is required to provide only leakage current. When a GSM mobile terminal is waiting for a call, paging mode, the terminal may be active less than 0.1% of the time. To provide maximum battery life the power supply for a GSM mobile must be highly efficient while sourcing leakage current and while sourcing operating current.
In the past, typically, low dropout voltage regulators (LDOs) were used to provide multiple stable supply voltages in battery-operated applications. However, standard LDOs exhibit poor efficiency as the difference between input and output voltage is increased. Recently, a number of DC/DC converter architectures have been proposed for portable systems to improve the efficiency of portable systems. These DC/DC converters tend to have low efficiency at the extremely low current loads (<200 μA) associated with leakage conditions. Additionally, these DC/DC converters tend to have either large output voltage ripple or slow voltage transient response. A linear regulator is often required either to provide a stable supply at extremely low current loads, to provide additional rejection of power supply noise, or to provide additional rejection of load noise.